Cable bolt

ABSTRACT

The present invention relates to the field of bolts, bars, wires, anchors and similar devices used for example as ground or rock support, reinforcement and anchors in geological environments such as mines, tunnels, etc. It also relates to stabilization and/or reinforcing applications for other geological ore earthwork applications. The invention is a cable bolt comprising at least two wires, and being adopted to have a nut threaded directly onto at least one of the wires. When the cable bolt is a multistrand (steel) cable, the outer wires (11) have a thread (13) formed upon them for a nut to engage. Alternatively, instead of a thread, a pattern of deformations could be used, then a nut or other member being locked onto these deformations. Preferably, the threads or deformations are rolled into the wires so that material is not removed and work hardening occurs.

FIELD OF INVENTION

The present invention relates to the field of bolts, bars and wires andsimilar devices used for example, as ground or rock support andreinforcement in geological environments including underground mines ortunnels or other stabilisation applications and also more generally toreinforcing applications. The present invention also relates to endfittings or means for securing the bolts, bars or wires.

BACKGROUND ART

Numerous examples exist of types of rock or ground stabilisation boltshaving the form of a rigid bar. The rigid bar generally has an elongatedshank for insertion in a borehole drilled from an excavation intosurrounding rock, which is to be contained or stabilised. The installedbar acts as a rock bolt, which together with a plate and nut provided atone end of the bar serve to reduce the risk of collapse of the rockforming the roof or walls or uplift of the floor of the excavation.

The borehole is usually drilled to a depth so that one end of the rigidbar and at least a portion of the length of the bar adjacent to this oneend is secured to relatively stable rock by a fast setting resin mix,other grout formulation or mechanical anchor device.

Such rigid bars are often of limited use where a borehole must bedrilled deep into the roof of the excavation before relatively stablestrata is located or where thicker zones are to be reinforced. The rigidbars are relatively inflexible, and thus a bar of greater length thanthe height of the mine or tunnel or any other type of excavation cannotbe installed without being plastically deformed and then straightenedagain before being inserted into the borehole. Rigid bars of aparticular diameter also have a relatively limited load carryingcapacity and therefore a relatively large number of rigid bars must beused over any given area to achieve the required support or reinforcingaction.

A cable form of rock bolt is shown in German Patent ApplicationDE3435117A. The cable form of rock bolt disclosed therein has a rigidend or sleeve portion formed at the end of the cable part of the bolt toenable a plate and nut to be fitted to the bolt. The rigid end isusually preformed on the cable by casting or swaging for example, andtherefore the cable bolt is provided in a predetermined length.Accordingly, a cable bolt must be ordered and supplied to the excavationsite, depending on the borehole depth. This is often not practical,where the depth of boreholes needs to be varied from area to area.

Another cable form of rock bolt is disclosed in U.K. PatentSpecification No. GB2084630A. The cable disclosed therein has ananchored swivel at one end of the cable which is inserted into theborehole in order to secure the bolt. At the other end of the bolt thereis provided a portion of rigid bar onto which a plate and nut can befitted. In manufacturing the rock bolt GB2084630A difficulty isencountered in attaching a rigid bar to the cable and also relativelyhigher costs are involved in its manufacture. Problems similar to thatof DE3435117A with regard to varying borehole depth equally apply inrespect of the bolt disclosed in GB2084630A.

A further problem encountered with rigid bar bolts as noted above istheir limited load carrying capacity per unit bolt diameter. When therigid bar bolt is in situ, the load of the rock forming the immediateroof of the excavation which is to be supported is transferred to therigid bar or known cable form via a plate by means of the threaded areabetween the nut and rigid end of the known bolts.

Devices of this general type which are inserted into drillholes andbonded to the rock are subject to possible axial forces and shearforces, the latter occurring as a result of at least partial sidewaysmovement of certain rock zones. Thus, to prevent premature yielding ofthe device when rigid bars are used, there is a tendency to use bars ofgreater diameter. However, this necessitates use of a heavier and moreexpensive bar and requires a larger diameter borehole to be drilled intothe rock. It would be seen of advantage to keep the diameter of theexposed end of the bolt small because small holes are more suited formaximum drilling speed and to form a small annular zone between theborehole and the bolt for efficient resin mixing and maximum bondstrength development. It would be an advantage to provide a cable rockbolt which is able to carry larger loads than that of known rigid barsof the same diameter so that borehole diameters and time of drilling andinstallation can be kept to a minimum.

It has also been found to be sometimes difficult to agitate resins inthe borehole to ensure correct mixing of constituents due to thesubstantially cylindrical nature of some prior art bars.

OBJECTS OF INVENTION

An object of the present invention is to alleviate some of the problemsof the prior art.

A further object of the present invention is to provide a cable bolt forearth or rock stabilisation which is adapted for fitment into a boreholeirrespective of its depth.

A further object of the present invention is to provide a cable boltadapted for use with relatively small diameter holes.

A further object of the present invention is to provide a cable boltwhich is adapted to carry relatively larger loads.

A further object of the present invention is to provide a means ofagitating resin in a borehole in association with a cable bolt.

A still further object of the invention is to provide a method ofsupport with the end of each support formed simply including formationat the face on segments of cable taken from a reel attached to anautomatic support placement machine.

SUMMARY OF INVENTION

The present invention provides a device adapted for rock or earthstabilisation and reinforcement. The device is provided in the form of asingle stranded cable or cable bolt. The cable bolt of the presentinvention is adapted to have a nut fitted directly onto one end of thecable. There is no need to have pre-threaded cables. The presentinvention enables fitment of the nut directly onto the cable. The cablemay be cut, in situ, to any desired length, and have a nut fitteddirectly to an end of the cable. In this way, cables or rigid bars offixed length are therefore no longer required.

The present invention further provides a cable bolt which comprises aplurality of wires. One end of the cable bolt is adapted to have athread rolled thereon. A nut placed on the threaded portion of the cablebolt serves to interengage the wires of the cable. This allows load tobe transferred to each wire of the cable. The cable bolt is thereforeadapted to carry relatively larger loads than known bars with rigidlyformed ends.

The wires of the cable bolt of the present invention may be interwound,bunched or otherwise arranged. In a preferred form of the presentinvention, the wires are parallel laid although cross lay may also beutilized. The contact areas between wires of the bolt thus extend alongthe surface of each wire for the entire length of the cable. The presentinvention further provides a cable bolt, formed of a plurality of wires,which has a relatively dense construction of wires in strandcross-section. Filler wires may also be provided in between outer andinner lays of wires, to provide an even greater area for the transfer ofload from the nut to the cable wires.

The present invention also provides a cable bolt, the outer wires ofwhich are wound with a lay direction opposite to the screw direction ofthe thread or spin direction of the cable. A lay direction opposite thethread direction is preferred, however, a lay direction the same as thethread direction can also be used in the present invention. The cablebolt of the present invention may advantageously be installed in aborehole together with a resin/grout cartridge. The lay direction of theouter wires as noted above provides a number of advantages. Oneadvantage is that after a nut is threaded onto one end of the cablebolt, the cable bolt is usually made to rotate until the resin in theborehole around the other end of the cable bolt sets. The lay directionbeing provided in a direction opposite to the screw direction of thethread, or spin direction of the cable, serves to cause a pumping actionon the resin in the borehole, and pumps the resin toward the closed endof the borehole. This pumping action serves to agitate and mix the resinbefore it sets.

Another advantage provided by the lay direction of the outer wires isthat it serves to reduce de-lamination of the wires of the cable bolt asa result of threading the nut onto the cable. The lay direction alsoserves to lock up the outer wires as they are rotated in the threaddirection during rolling of the thread and enables a consistent threadto be formed on each outer wire of the strand.

The present invention provides a cable bolt comprising at least twowires, the bolt being adapted to have a nut threaded directly onto atleast one of the wires. The cable bolt may have the at least two wiresinterwound.

The present invention also provides a cable bolt comprising a centralwire and an outer layer formed of a plurality of wires wound about saidcentral wire, a thread form formed directly onto the wires in said outerlayer at one end of the cable bolt, said thread form being adapted tohave a nut threaded thereon.

In one form the cable bolt may have at least one intermediate layerprovided between said central wire and said outer layer, the wires insaid intermediate layer or layers and said outer layer being woundaround said central wire in a predetermined lay direction with the wiresin each layer being substantially parallel to one another.

The thread form may be rolled onto the wires of said outer layer.

The thread form may be rolled in a direction opposite to the laydirection of the outer layer.

The free ends of the wires located at one end of the cable bolt may besecured to one another, for example, by welding.

The formation of the thread may serve to interengage wires forming saidcable bolt.

The present invention also provides a method of installing a cable boltin a rock or earthen formation, said method comprising the steps of:

forming a borehole in said rock or earthen formation;

placing a settable securing material cartridge in said borehole followedby cable bolt material from a storage facility for said cable boltmaterial;

separating a predetermined length of said cable bolt material from saidstorage facility and securing ends of wires of the cable bolt materialat a free end of the cable bolt material;

rolling a thread form on said free end of the cable bolt material;

applying a plate and a retaining nut to the thread form on said cablebolt material;

rotating said cable bolt material to activate said securing materialcartridge; and

once said securing material has set, tightening said nut on said threadform. The present invention may further provide a cable adapted to useas a cable bolt, said cable comprising at least two wires. The presentinvention still further provides a nut adapted to radially compresswires of a cable bolt. The nut may have at least one axial slot therein.The present invention still further provides in combination, a cablebolt comprising at least two wires, an outer surface of the cable bolthaving at least one depression formed therein; and

an end fitting adapted to co-operate with said depression whereby in useremoval of the end fitting from the cable bolt by axial movement only issubstantially prevented. The depression may be formed by a groove in oneof the wires. The present invention also provides a method of providingan end fitting on a cable bolt, said method comprising the steps of:

a) providing at least one depression proximate an end of said cablebolt, said depression being adapted to co-operate with said end-fitting;and

b ) installing said end fitting directly onto said cable bolt in amanner in which the end-fitting is substantially held in place on saidcable bolt.

A preferred embodiment of the present invention will now be describedwith reference to the accompanying drawings, wherein like numerals areused to refer to the same component parts, and wherein:

FIG. 1 shows a right hand lay cable and a fight hand threaded nut, beingone form of cable bolt of the present invention installed in a borehole.

FIG. 1A shows a left hand lay cable and an alternative form of retainingnut;

FIG. 2 shows in cross-section, a preferred form of cable bolt inaccordance with the present invention.

FIG. 3 shows in section, a left hand lay cable and the threaded end of acable bolt in accordance with the present invention, with a nut inplace,

FIG. 3a is a view of the threaded end of a cable bolt taken along lines3a of FIG. 3,

FIG. 4 shows a preferred method of manufacturing and installing a cablebolt in accordance with the present invention.

FIG. 5 shows one form of one nut.

FIG. 5a shows a top view of the nut shown in FIG.5.

FIG. 6 and 7 show examples of collars and plates.

FIG. 8 shows one form of conventional nut.

FIG. 8a shows a side view of the nut shown in FIG.8.

FIG. 9 shows diagrammatically the present cable bolt used as an earthenor rock stabiliser.

FIG.10 shows diagrammatically the present cable bolt when subject tolateral movement;

FIG.11 shows graphically a representative comparison of holding betweenthe present cable bolt and prior art rigid bar; and

FIG. 12 is a table showing preferred strand cross-sections and diameterranges for the cable bolt.

The present invention provides a cable bolt, which has numerousapplications, for example in building or civil construction, rock andearth stabilisation and/or reinforcement, or any other application whichcurrently involve the use of cables or rods as fixing elements or asreinforcement.

A preferred embodiment of the present invention will be described withregard to an application in earth or rock stabilisation. The presentinvention should, however, not be seen as being limited to such anapplication. For example, the cable bolt may be used in a supportingfunction, FIG. 9, in which the cable bolt 6 may be substantially fullyencapsulated by resins in a bore hole 4. In this way, the bolt may actto reinforce an unstable portion of earth 2 and enhance its strengthproperties so it becomes self supporting.

Furthermore, although the present invention is disclosed in theembodiment with only one threaded end, it is to be understood thatapplications exist where both ends of the cable bolt can be threaded ina similar fashion to the one end described, to receive a nut.

Thus, with reference to an application of the present invention in thefield of earth or rock stabilisation, and in particular a mining ortunnel excavation, FIG. 1 shows a roof section 1 of a tunnel. The rockabove and forming the tunnel roof 1 comprises, for example, a relativelyunstable portion 2, and a relatively stable portion 3.

In such situations a cable bolt according to the present invention isinstalled, to reduce the risk of the unstable portion of the tunnelcollapsing.

A borehole 4 is drilled into the tunnel roof, or wherever the earth orrock requires stabilisation, to a depth which enables one end of thecable bolt to preferably be fixed to the more stable portion 3. Eachborehole depth may vary from hole to hole, depending upon the locationof a suitable portion.

Grout 5 is inserted in the borehole 4, in a manner known to the skilledperson, and the cable bolt 6 of the present invention, shown of lengthgreater than the length of the borehole to enable a nut and plate to befitted on the exposed end, is thereafter inserted into the borehole.There are situations where grout 5 would be inserted after the cablebolt 6.

A threaded portion may be formed prior to or subsequent to installingthe cable bolt. It is usual practice however, in the art to form thethread prior to installation of the cable bolt. The threaded portion ispreferably formed by rolling. It is believed that thread cutting wouldremove metal from the outer wires of the cable and reduce the loadcarrying capacity of the cable bolt whereas rolling deforms the metaland creates a raised edge which protrudes slightly above the preformedsurface of the outer wires. The deformation is also believed to workharden the outer wires thereby increasing their, strength which partlycompensates for the reduced cross section area caused by thread forming.

In installation, a plate 7 is placed on the cable bolt 6, and then a nut8 is threaded onto the cable bolt to hold the plate 7 against the tunnelroof 1.

As described above, the plate 7 serves to hold the unstable portion 2 inplace by reducing its ability to break away from the stable portion 3.The purpose of the plate, should be to transfer any surface rockmovement into stretch in the cable which results in a resistance forcebeing generated in the cable which acts on the plate and which resistsfurther movement of the surface. More details of the load transfer willbe hereinafter described with reference to FIG. 3.

FIG. 2 shows one form of cable bolt in accordance with the presentinvention. The cable bolt has one king or central wire 9, an inner layerof five wires 10, an outer layer of ten wires 11, and filler wires 12placed between the outer and inner layers.

It is important to note that FIG. 2 shows only one exemplary form ofthe. present invention. The present invention may comprise any number ofwires, strands, ropes and cables, depending upon the application.

is to be noted that, in cable cross section larger load carryingcapacity may be provided by forming the cable of a relatively largenumber of wires, each wire having relatively high strength. The use of aplurality of wires enables each wire to carry a portion of the load.

STRAND GEOMETRY

Strand geometry can be selected according to the following criteria:

outer wire diameter needs to be sufficiently large so that thread orgroove indentations do not exceed 20% of outer wire diameter and toprovide sufficient flexural rigidity for the strand; experience hasindicated that outer wires in the diameter range 5.0 to 5.5 mm arepreferred;

given the above requirement for outer wire size, the number of outerwires depends on the strand diameter required; and

core wires, if appropriate, and the central wire of the strand mustpreferably have a diameter that will allow them to be formed into a"close packed"structure (i.e. each core wire has as many contacts aspossible with other core wires, the central wire and the outer wires).Note that to achieve a close packed structure, a parallel lay strandconstruction is required. However, it is also possible to have across-lay construction in which the outer wires are wound with a laydirection opposite to the core wires, as herein disclosed.

Examples (only) of preferred strand cross-sections and diameter rangesare shown in FIG. 12. These are typical examples of size ranges thatwould be suitable for the cable bolt when it is used for fully bondedrock support/reinforcement installed with resin cartridges. Many othertypes and/or forms of cable bolt are contemplated in accordance with.the application to which the bolt is to be subjected. The presentdescription is to be used by an artisan as a guide to theconstruction/configuration of other types and/or forms of cable bolt.

Referring to FIG. 2, one form of cable bolt as described above, hasapplication in the mining field.

The dimensions and make up of the particular strand cable that may beused are as follows: a central king wire is 3.80 mm in diameter, kingwire is surrounded by five (5) wires each 4.53 mm in diameter, five (5)filler wires of diameter 2.1 mm are used in the outer grooves betweenthe 4.53 mm diameter wires, and ten (10) wires 4.9 mm in diameter arewound around the outside.

The outer diameter is approximately 23.1mm.

Noting the above, trials of the cable of one form of cable bolt haveshown: the outer wire diameter should be as large as possible compatiblewith the outer strand diameter required and flexibility (i.e. bendingstiffness). For a strand with diameters in the range 22.8-23.3 mm, adesign with ten (10) outer wires has been found to allow a low enoughbending stiffness for mining ground support applications. Similarly, astrand with a diameter range from 15.2 to 16.0 mm with six (6) outerwires is still flexible enough for the above purpose. With both thesesize ranges, the outer wire diameter is preferably in the range 5.0 to5.5 mm.

All wires in the strand except the centre (or king) wire should be woundin parallel lay with a lay direction opposite to the screw direction ofthe thread.

The cross sectional area within the core of the strand (i.e. the areabounded by the total number of outer wires arranged in their radialposition) is to be as tightly packed with wires as possible. This isrequired to maximise the number of radial contacts for each wire in thecore and to maximise the radial compressive stiffness of the core. Thebreaking strength of the cable is partly dependent on the ultimatestrength capacity of the wires selected for the core.

The above are considered to be important where the thread is rolled onthe outer wires. A rolled thread is preferred unless the outer wires aresufficiently large enough to enable thread cutting, as it is usually notpossible to achieve adequate thread depth for load transfer purposeswithout excessively weakening the outer wires if the thread form is cutinto the wires. In other words, there may be an optimum condition ofthread depth and outer wire diameter at which the outer wire strength isequal to the failure strength of the thread when a nut of a specificlength is used.

An indentation in an outer wire may otherwise be provided, theindentation co-operating with a suitable end fitting. For example, theend fitting may simply be clipped onto the end of the cable bolt, wherea protrusion of the end fitting co-operates with the cable indentation.

It is preferred that the core is densely packed with wires. The cablebolt of the prescribed invention in conjunction with a cone nut or tightfitting conventional nut utilises the phenomena of the nut compressingthe outer wires onto the inner core wires which may in turn becompressed onto the king wire to develop sufficient friction between thewires, so that, for example, as the outer wires stretch under load, theinner wires also stretch and build up tensile load. If this does notoccur, the tensile strength of the cable bolt is only that of the outerwires, and reduced load carrying capacity results.

For increased load capacity of the threaded cable it is preferred thatthe cable be formed by winding the wires around the central king-wirewithout using lubricants of any kind (rope manufacturers often usegrease during the manufacturing process for corrosion protection duringthe life of the product). Where lubricants are used, premature slippagemay result between inner and outer wires.

When a cone nut is used, it is preferable that the outer wire diameteris selected to allow a small space between each outer wire. This allowsthe nut to squeeze the outer wires onto the inner core wires moreeffectively and assist in the loead transfer to the inner core wires.This is not always the case with a parallel (conventional) nut. Thesqueezing action is considered not to be essential to the working of thepresent invention where there are small spaces between each outer wire,these gaps also allow the grout or glue used to bond the strand to therock (portion 3 of FIG. 1) in a borehole to penetrate the voids betweenouter and inner core wires thereby increasing the bond strength.

Where the load capacity of the threaded strand/cone nut assembly is tobe close to the maximum and/or at least 80% of the nominal breakingstrength of the strand, none of the wires used to construct the strandshould be coated with anti-corrosive layer (such as galvanising). Thesecoatings tend to reduce the radial stiffness of the strand and serve toprovide a lubricating effect on the wire surfaces when in contact witheach other. Both these aspects tend to detract from the frictional loadtransfer between the outer and core wires. Coatings which maysignificantly increase friction may be an advantage.

FIG. 3 shows, in cross section, the interaction of wires of the cablebolt of the present invention. It is to be noted that, although central,inner and outer wires are shown of equal cross-sectional area, the wiresof the cable bolt may be of any varying cross-sectional area in order toachieve a desired strength capacity.

The central (king) wire is shown as being straight.

A rolled thread 13 is provided on the outer layer of wires 11. Therolling of the thread has the added effect of engaging the wires of onelayer to the wires of another layer. Deformations 14 may be formed wherethe wires are compressed together, in the case where a cone nut is used.

Interengaging of these deformed areas serves to improve load carryingability of the cable. These contact areas 14 serve to transfer ordistribute the load applied to nut 8 to the wires of the cable bolt, andtherefore increase the load capacity of the cable bolt.

In addition to the interengagement of the wires noted above, acompression nut (for example the nut shown in FIGS. 1A or 5) or a nutwhich provides an interference fit with the cable bolt, may serve toprovide compressive forces radially on the wires. The slots formed inthe nut may be configured to allow compression of cable wires as the nutis tightened. The slots may be oriented axially and/or radially. Also,the cone section may be separate to the nut and be engaged by the nut torotate both cone and nut. The slots may also allow be configured toallow for movement of the plate and collar in an axial direction.

As shown in section A--A, where the wires are deformed at theirinterengaged surfaces during rolling the wires increase the area andextent of their contact. Where the wires are not deformed, theypreferably are arranged to engage each other. Thus wire 11 engages innerwire 10 at 14a and also engages filler wire 12 which in turn engagesinner wire 10 at 14b.

Inner wire 10, likewise deforms and interengages its neighbouring wires,and in particular king wire 9 at 14c. As is shown, each wire of thecable, in this example, is slightly and locally deformed by the threadrolling process to increase contact area between itself and itsneighbouring wires. This serves to assist in distributing the load fromthe nut, to each wire of the cable bolt.

The nut 8 design depends on the load capacity desired. Preferably, thethread matches the form of the rolled thread on the outer wires. Asshown in FIGS. 1, 3 and 8, the nut may be of conventional shape andlength if adequate load transference can be achieved thereby. Forexample, the nut as shown in FIG. 8 in conjunction with a 23.1 mmdiameter cable bolt has been tested to transfer capacity as follows:

    ______________________________________                                        Nut load transfer capacity (tonnes)                                                                Nut length (mm)                                          ______________________________________                                        20                   30                                                       26                   36                                                       30                   42                                                       35                   48                                                       ______________________________________                                    

The nut can transfer a minimum force equivalent to the strength of theouter wires. If there is some wire interaction, for example by frictionor wire compression, the transfer force can be increased. If improvedload transference is needed, the nut as shown in FIGS. 1A and 5 with afrusto-conical end piece 20 might be used. The end section 20 hasconveniently two sets of diametrically opposed axial slots 21 to allowthe opposed regions of the end section 20 to be compressed against thecable as the nut is threaded thereon and is screwed into a complementarytapered opening 22 in the collar piece used in association with a plate.Particular collar and plate embodiments are shown in FIGS. 6 and 7. A 7°taper on the cone used in conjunction with a collar with a 7° taperedhole with 3 mm wide slots in the cone allows the opposed regions of theend section 20 to provide adequate compression when the nut in FIG. 5 isused in conjunction with a 23.1 mm diameter cable bolt. The collar inFIG. 6 has a spherical surface machined on part of its outer surface tolocate and bear on a deformed plate as shown. An advantage of thisarrangement is that it allows for some plate misalignment from a planewhich is perpendicular to the axis of the belt. In situations wherebolts are installed perpendicular to the rock or earth surface, acylindrical shaped collar in Figure 7 can be used in conjunction with aflat plate. Collars of the type shown in FIGS. 6 and 7 manufactured frommedium strength steel provide sufficient confinement of the nut in FIG.5 if the collar outside diameter is at least 50 mm and the length is atleast 22 mm.

Furthermore, the rolling of the thread is preferred as this deforms themetal of the wires so there is a reduction in cross section area of theouter wires of the cable bolt, but this is compensated to a degree bythe extra strength in the wires due to work hardening proximate thethreaded area. Forming the thread in this way obviates the need to use arigid bar and alleviates a prior art problem where there may bepremature yield of a rigid bar subjected to shear deformation.

FIG. 10 illustrates the typical profile that a rock bolt is subjected toafter shear movement in the rock has occurred. A rigid bar bolt of theprior art has been found to be forced to yield and fail after arelatively small shear movement, whereas in the cable bolt of thepresent invention localised movement between individual wires occurs toallow relatively high shear movement before wire failure occurs.

Tests of the present cable bolt have also shown that if the end of thecable bolt moves or is pulled out of a stable zone, an increase in theholding force of the cable bolt in the borehole develops. A rigid barhas been found to merely slip out of the borehole in this situation.FIG. 11 diagrammatically illustrates a comparison between the presentcable bolt and a rigid bar in such a situation.

With reference to FIG. 4, a preferred method of utilising the cable boltof the present invention is described.

The steps are as follows in this preferred example, however, thefollowing steps are not applicable in all installations of the cablebolt in accordance with the present invention:

(a) Forming a borehole into the excavation rock which is to bestabilised by a cable bolt in accordance with the invention;

(b) Cutting a length of cable (V) from a drum of cable either afterfeeding cable from the drum into the borehole or prior to insertion ofthe cable into the borehole. In either method the length is cut to suitthe depth of the hole of step (a) above.

(c) At least the end of the cable on to which the thread is to be rolledis welded (W) to hold the ends of the wires together thereby reducingthe likelihood of delamination of the wires of the cable. If desired,some other mechanical or other known method could be used to securethese wire ends together.

(d) Rolling a desired thread form onto the end of the cable lengthsecured together by welding or by some other means such that the outerwires of said cable become locally delaminated in the area of the threadrolling where the outer diameter of the cable increases slightly fromthe welded or otherwise secured end.

(e) Placing a stabilisation plate with an opening therein over theprojecting end of the cable length in the borehole.

(f) Threading a nut onto the projecting end of the cable length untilsuch stage as the nut stiffens on the thread as a result of the expandedcable diameter or until the end of the cable contacts the pin placedacross the threaded portion of the nut as shown in FIGS. 5 and 8.

(g) Installing the cable length into the borehole in the rock face ifthis has not already occurred. Generally, the cable is installed with aplate and nut already fitted. The nut is used to spin the bolt duringinstallation.

(h) Rotating the nut and cable together so as to break a fast settingresin cartridge pre placed in the borehole and to thoroughly mix theresin materials to secure the inner end of the cable to the adjacentrock wall within the borehole. The arrangement of the wound wires of thecable having a lay direction opposite to the thread rolled thereon, whenrotated in the direction of the thread, provides a pumping action to theresin materials so that they tend to move inwardly within the boreholerather than outwardly therefrom while the resin remains liquid.

(i) When the resin has set, the nut is then forced onto the thread ofthe cable end to fail the pin and to press the stabilisation platefirmly against the rock face. Alternatively, it is envisaged that theouter wires can be welded together and thereafter a thread rolled oneither side of the weld. The cable may be then cut through the weldedsection. In another alternative, the cable (or a portion thereof) may bethread rolled. first, after which the cable may be cut to a desiredlength.

BENDING STIFFNESS

In order to successfully install the cable bolt by spinning it throughone or more resin cartridges, the strand must have sufficient flexural(bending) rigidity so that it does not bend when the thrust is appliedto the end of the bolt during installation. This property of the strandis primarily a function of the number of outer wires, the outer wirediameter and the radial distance of the outer wires from the centrewire. Single strand cable bolts of the configurations and diameter shownabove have sufficient flexural rigidity to be installed by the methodindicated in the specification.

OUTER WIRE INDENTATIONS

Although the specification as it now stands covers indentation of partof each outer wire so that a thread is formed, rolled or cut around thestrand, the indentations need not necessarily be arranged to form athread. The combination of successive indentations around the outerwires to form a thread allows a threaded nut to be used as the "endfitting" to bear against a collar and/or plate.

Indenting the outer wires in this way is only one particular form ofdeforming the outer wires. Provided other types of end fitting could beused, the outer wires could be rolled with a set of parallel groovesnormal to the strand axis (centre wire). Groove dimensions in each outerwire would be the same as for the case when a thread is formed on theouter wires of the strand. With the parallel groove type of indentation,the end fitting would need to be swaged or crimped onto the strandduring manufacture and have an external shape (at least on the drivenend) to allow it to be spun and hence spin the bolt during boltinstallation. This end fitting would not allow the bolt to be tensionedduring the installation process. The end fitting may be formed to simply"snap-on" to the end of the cable bolt.

Other forms of cable are also contemplated, such as a cable formed ofnon-round wires. The wires may be of trapezoidal, elliptical ortriangular shape. These shapes may provide a more consistent thread,greater inter-wire contact area for load transfer and therefore higherload carrying capacity. The wires may also be formed with crosssectional shapes so as to interact in a half locked coil or full lockedcoil manner.

Although the present description discloses a cable bolt of a strandconfiguration, a cable bolt of a rope configuration is also hereincontemplated.

We claim:
 1. A cable bolt comprising:a central wire and an outer layerformed of at least one wire wound about said central wire; a thread formformed directly onto the wires in said outer layer at one end of thecable bolt, said thread form being adapted to have a nut directlythreaded thereon; and at least one intermediate layer provided betweensaid central wire and said outer layer, the wires in said intermediatelayer and said outer layer being wound around said central wire in apredetermined lay direction with the wires in each layer beingsubstantially parallel to one another.
 2. A cable bolt as defined inclaim 1, wherein said thread form is rolled onto the wires of said outerlayer.
 3. A cable bolt as claimed in claim 2, and further comprising:anut threadably engaging said thread form, and squeezing the wire of theouter layer onto the wire of the intermediate layer, and squeezing thewire of the intermediate layer onto the central wire, such that uponloading of the nut, the outer, intermediate, and central wires arestretched.
 4. A cable bolt defined in claim 1, and further comprising:anut adapted to squeeze the wire of the outer layer onto the wire of theintermediate layer, and to squeeze the wire of the intermediate layeronto the central wire, such that upon loading of the nut, the outer,intermediate, and central wires are stretched.
 5. A cable bolt asdefined in claim 1 wherein:said predetermined lay direction is oppositethe direction of the thread form.
 6. A cable bolt, comprising:at leasttwo wires bound together to form a bolt, with at least one of the wireshaving an exposed outer surface; and a plurality of indentationsdirectly formed on said exposed outer surface and defining a threadedscrew portion onto which a nut may be directly threaded, saidindentations comprising deformations roll formed into said exposed outersurface of at least one of the wires.
 7. A cable bolt comprising:atleast two wires bound together to form a bolt, with at least one of thewires having an exposed outer surface; a plurality of indentationsdirectly formed on said exposed outer surface and defining a threadedscrew portion onto which a nut may be directly threaded; and said atleast one wire is wound with a lay direction opposite to the screwdirection of said threaded portion.
 8. A cable bolt comprising:a centralwire and an outer layer formed of at least one wire wound about saidcentral wire; a thread form formed directly onto the wires in said outerlayer at one end of the cable bolt, said thread form being adapted tohave a nut directly threaded thereon, and said thread form being rolledonto the wires of said outer layer.
 9. A cable bolt as claimed in claim8, and further comprising:a nut threadably engaging said thread form andsqueezing the wire of the outer layer onto said central wire, such thatupon loading of the nut, the outer and central wires are stretched. 10.A cable bolt comprising:a central wire and an outer layer formed of atleast one wire wound about said central wire; a thread form formeddirectly onto the wires in said outer layer at one end of the cablebolt, said thread form being adapted to have a nut threaded thereon; anda nut adapted to squeeze the wire of the outer layer onto said centralwire, such that upon loading of the nut, the outer and central wires arestretched.
 11. A cable bolt and nut combination, comprising:at least twowires wound together to form the cable bolt, with one wire being aninner wire and the other wire being an outer wire and having an exposedouter surface; a plurality of indentations directly formed on the outersurface of said outer wire to define a threaded portion, said outer wirebeing wound with a lay direction opposite to the screw direction of thethreaded portion; and a nut rotatably threaded onto said threadedportion such that a load can be placed on the nut.